Monday, February 18, 2008

6. Sequence of Events for TTL Flash

Let's assume we have these initial conditions:
normal indoor ambient lighting, camera A mode at f/3.5 and ISO 400.

Flash in Hotshoe

When you shoot a flash picture with the flash in the hotshoe, the following flash sequences take place:

1. Flash fires its preflash sequence.

2. The flash metering system measures the reflected light in the center of the frame from the preflashes and determines the power for the main flash.

3. The calculated power for the main flash is then modified by flash compensation that is set on the on the camera or the flash. This includes the dedicated flash compensation button on the camera, the camera ev setting, and the compensation setting on the flash itself. The power for the main flash is now finalized.

4. The shutter opens and the flash fires its main flash.

Remote Wireless Flash

Now, when the flash will be fired wirelessly a lot more commuication takes place during the preflash sequence. Here are the steps:

1. The Commander fires a control flash sequence that tells each of the remote flashes to fire its preflash sequence.

2. Each remote group fires its preflash sequence one at a time. If there are multiple flashes in a group, they will all fire their preflash sequence simultaneously.

3. Based on some very valuable work by Hal Becker (my fellow Moderator 'HBB' at Nikonians Speelight Forum, we now know that the flash metering system reads the reflected light from each group individually and immediately calculates and sends power information back to each group. Note that this system is not as smart as we originally thought. Each group's power is set irrespective of other groups that may be in use, and there is no adjustment for a situation where multiple flashes hit the subject in the same spot. I believe that this is why each group alone is purposefully set to underexpose the subject. Then, when multiple flashes are used, like in a studio, the subject is usually exposed properly.

A power setting is also set into the commander unit if it contains a flash as well. Multiple flashes in the same group will all receive the same power settings.

4. The shutter opens.

5. The Commander fires another control sequence telling all groups to fire at the power decided in step 3.

Also, the preflash sequence gets visibly longer when more remote groups are being controlled, and it becomes more important to block the visible light from the commander to avoid the preflash causing the subjects to blink.

If you want to see the preflash separately from the main flash, use the FV Lock button. You will see that when the flash is in the hot shoe, the preflash sequence is very short. As soon as you use the flash wirelessly, however, the preflash sequence gets visibly long due to all the extra communication that must take place in this mode.

Also the preflash sequence delay can cause problems when shooting action, so it is best to use FV Lock first which removes the preflash delay.

Next: How to Shoot Large Groups using Nikon CLS


Raymond said...

Hi, Russ, you mentioned here that "The calculated power for the main flash is then modified by flash compensation that is set on the on the camera or the flash. This includes the dedicated flash compensation button on the camera, the camera ev setting, and the compensation setting on the flash itself. The power for the main flash is now finalized."
Are these cumulative?

Russ MacDonald said...

Hi Raymond,

Yes, they are cumulative. I always leave the flash ec button on the camera set to 0 to reduce confusion and do all of my flash ec on the flash itself.

The camera ev button also adjusts the ambient contribution, so I only use this if I want to change the entire image brightness.

Also the camera ev adjustment doesn't work predictably with the flash in TTL-BL mode. It adjusts the ambient properly, but it appears to interfere with the balancing equations that are being solved by the flash exposure computer. Sometimes the flash power changes correctly and sometimes it doesn't.


Anonymous said...

Thx for good description, hard to find specs on this. Q1/ Using FVlock, are the first two steps executed so we start with the 3rd command sequence? Q2/ How many ms delay using FVL and 2 channel remotes? If less than 100ms, should be no problem with fast blinkers ...

Russ MacDonald said...

Hi brikah,

No. After pressing FV Lock, the flash power is locked into each remote, so the sequence would start at 5.

So, when using FV Lock, the only delay is the from the command string that tells the remotes to fire.

I have not seen a spec on the maximum delay for preflash pulses, but I can tell you that some people can indeed blink fast enough so their eyes are closed when the units fire their main flashes.

This is especially common when shooting certain animals, and FV Lock helps a lot. For some fast birds, like humming birds, you have to use SU-4 manual mode on the SB800, which eliminates all delay from preflash pulses.

Of course, one of the main advantages of FV Lock is that all your pictures have exactly the same exposure.


TBG said...

brilliant work. having used pocketwizards for so long, i never gave ttl or cls much mind. Got my hands on some radiopoppers, (which im pretty sure youd be interested in .. google em) and had to learn how cls/ttl actually works. nikon does a horrendous job of explaining, but you have done a masterful job. thanks again

Russ MacDonald said...


Thanks for the nice feedback!

I am aware of the Radio Poppers, and would like to try them, but they are very expensive.

Plus, I normally don't have any problem getting the Nikon Wireless to work well.


Anonymous said...

Hi Russ

Firstly, great blog, very informative.

I use my SB800's off camera, usually with a shoot through umbrella. If I move the umbrella closer to the subject or further away the exposure on the subject is perfect...amazing stuff.

Now, I don't really understand how TTL accomplishes this. If the flash fires a preflash it will hit the umbrella and not record an accurate light read...right? I always thought the flashes just stay on until the camera determines it has enough light for a proper exposure at which point the flash shuts off. But from what I understand of your description the power output is determined before hand. So I thought if I moved the umbrella and light closer it would overexpose but it doesn't and the exposure is spot on.

In another setup I have the same umbrella setup described above but with an additional light illuminating the background, how does TTL determine the power level for that flash? When I did this actual set up the power level was just as I it read my mind or something :-)


Russ MacDonald said...

Hi Dave,

Thanks for the nice feedback!

What you have 'discovered' is the tremendous technical achievement that TTL gives us! You are exactly right. The system will always adjust itself to give the correct exposure no matter what you do to the flash (within the limits of the flash maximum power).

The reason this works is that the computer that is analyzing the light is in the camera - not in the flash. The amount of the preflash the the camera sees through its lens is what determines the power needed to achieve proper exposure.

The flash computer in the camera figures out what power the flash needs to fire at in order to light the subject properly.

Then, it sends the calculated flash setting back to the flash and the flash locks it in and waits for the final 'fire' pulse.

This allows you to put any modifier you want between the flash and the subject, and the flash computer in the camera will increase the power of the flash so that the subject is illuminated correctly.

The light that bounces back to the flash from the umbrella has no effect on the power calculation. It's the light that is reflected off the subject and through the lens that sets the power.

Please let me know if I have clarified this for you.


Anonymous said...

Ah, that makes perfect sense. Thanks for clearing that up!

Look forward to more blog posts.

All the best

Kendall said...

Hi Russ,

I was wondering if you could answer a CLS question I have in the back of my head. Is there an advantage to using a Nikon Speedlight for the commander (SB-900, SB-800) over using the built in flash on the D90 as the commander? Does the SB-800 have more range/power for communicating with remotes?

Have you seen NIKONSchool's DVD "A hands-on Guide to Creative Lighting" with Joe Mcnally? I just got it and watched it as was impressed. It was blew me away is some of the places that he put remotes, I just can't see how the commander signal would reach those lights. I just was wondering your thoughts.

I too am very grateful for the time you have spent in sharing your knowledge of CLS.

Thanks in advance,


Russ MacDonald said...

Hi Kendall,

Well, I can only speak about my SB-800 and D200, but I think the SB-900 and D90 are similar.

Yes, here are several advantages to using the external flash as a commander versus the pop-up flash:

1) Since the SB-800 is larger and more powerful, it definitely makes the entire CLS system work more reliably and at a farther distance.

2) The SB-800 can control three remote groups, while the pop-up can only control two.

3) The control panel on the back of the SB-800 is easier to use than the menus in the camera when you want to make changes.

4) When the SB-800 is in the hot shoe of my D200, the camera puts out a properly timed PC Sync if you want to trigger manual flashes with cords in addition to your CLS ones, whereas when the pop-up flash is used, the PC Sync output is disabled.

I haven't seen the Mcnally CLS DVD, but I know his work, and I am willing to bet it is a great video. He does wonderful things with CLS! I plan to get it soon.

Thanks for the kind feedback!


Anonymous said...

Wonderful, very helpful. thanks.

can you help me on the following for wireless SB800 exposure:

camera, A,f2.8, 1/60 (once use commander,--),

sb800, remote

the picture come out quite dark in dark room;

with camera, P, f2.8, others sames, the picture come out good;

can you help me explain what is going on?


Russ MacDonald said...

To Anonymous,

If I understand your question, I think you are asking why you are getting underexposure when in camera A mode and normal exposure in camera P mode, with the flash in remote mode, and the commander in --.

You should have gotten identical results in camera A and P modes. Something must have been different between the two pictures.

Maybe the subject wasn't centered the same in each one.

Maybe the subject in each one wore different colored clothing. Dark clothing will increase flash power, and light clothing will decrease it.

Maybe the flash was positioned at a different distance, or not pointed exactly the same in each picture. Sometimes when you are close to the maximum power of the flash, a slight change in its position will cause you to not have enough power and you run out of power leaving the image dark.

Maybe the flash was not fully recycled on the dark picture. When the batteries get close to depletion, the recycle time can get very long; maybe as long as a full minute. I always change batteries when the recycle time gets to 10 seconds.

That's all I can think of for the moment. Hope it helps.


Anonymous said...

Dear Russ

thanks for quick reply. very helpful.

I followed your suggestions on why camera A and P mode give different exposure under wireless mode.

i am not sure if my SB800 work correctly. even I took out the diffuser, SB800 cannot be zoomed (dont know why). Maybe it is the reason.

But i dont find one reason could be the SB800 red window is blocked somehow. I have one question on this:

you said that the camera will receive the preflash signal from wireless unit. if wireless SB800 placed 45 degree to subject, how the camera can receive the signal from pre-flash through the lens, because the preflash will reflect into another 45 degree, unless pre-flash is not light. very confused.

Russ MacDonald said...

Hi again Anonymous,

There are two things that will prevent the SB-800 from zooming. 1)the snap-on diffuser is attached, and/or 2) the built-in wide angle flash adapter lens is pulled out.

As long as neither of these things is true, it will match the zoom of the lens or zoom manually depending on the setting you make on the back of the flash.

Second subject:

You misread me. I did NOT say that the camera will "receive the preflash signal from the wireless unit".

I said the camera receives the REFLECTED light bouncing back from the SUBJECT caused by the preflash. It receives this reflected light through the lens (TTL) in the center weighted frame.


Anonymous said...

Dear russ

Even camera receives the reflected light caused by wireless unit, i just feel curious that the reflected light can be metered correctly because i thought 45 degree input light will be mostly reflected into 45 degree output, which leave very little light detected by camera.

i learned lot from your blog, thank again.

Russ MacDonald said...

OK, now I understand.

What you are talking about is a law of physics that says that the angle of incidence always equals the angle of reflection. And that is true.

However, what you are missing is that that law only applies to infinitely smooth surfaces.

A real subject is never perfectly smooth. The preflash light is partially diffused as it bounces off the subject, and some is reflected back to the camera. The part that bounces back to the camera is what is measured by the camera and used to set the flash power.


John Meyer said...

Great blog!

Anonymous said...

(first I must apologize due to my poor English)

Thank you very much for your tremendous blog. It is clear and remarkably argued.

When you talk about the study of Hal Becker, you write « the flash metering system reads the reflected light from each channel individually ». I perhaps misunderstood it, but I thought he was speaking of « groups », instead of « channels ». Did I make a mistake?

Best regards,

Russ MacDonald said...


Thanks you for your nice feedback, and you are absolutely correct! I used the word 'channel' instead of 'Group' incorrectly multiple times.

I immediately corrected my blog after reading your post.

Thank you for pointing that out to me! It's actually quite amazing that no one else caught that!



Anonymous said...


Very happy to contribute! (for just a little point, in fact...)

I have a question: do you intend to write a post about the SU-800?
Infrared control of remote flashes avoid preflashes (and subjects blinking), but it seems to have other limitations (range, angle of use) and, of course, an other protocol of communication with the remote flashes.

It should be great to have your analysis of this equipment!
Thank you in advance for your help.

Best regards,

Russ MacDonald said...

Hi Gilles,

I don't own an SU-800, and one of my personal rules is never to write about products I don't own.

I have read that the SU-800 has more range under some circumtances and less under others, but I can't verify that.

I do know that if you use an SB-800 as a commander, and you put a Gary Fong Light Sphere on it that you get 360 degree coverage so that remotes can be placed slightly behind the camera position. I don't think this would be possible with an SU-800.

I also know that you can tape an IR diffuser (Nikon SG-3IR) to the SB-800, and it will mostly eliminate the visible preflash, and the commands are still is diffused by the LS II just like when using visible light and units behind the camera work just fine.


Anonymous said...


Thank you for your answer (and for all your blog, of course!)

Best regards,

Daniel said...

Hi Russ,

Great Website, I still don't know why Nikon don't do more for their customers and provide some tutorials/FAQs for CLS as it can be quite complicated.

My question related to getting the best from the CLS/camera metering interaction. I am using a D200/SB600 combo and find that when I am using the SB600 wirelessly off camera with D200 pop up in commander mode, I seem to get 2-3 stops underexposure (more so when trying to light a subject whilst deliberately trying to underexpose the background). When the flash is on-camera I don't seem to get this issue or at least not as bad.


Russ MacDonald said...

Hi Daniel,

What you are seeing is normal, except it should be more like 1 to 2 stops instead of 2-3 stops. I actually wrote about this in my blog #6:

In summary, the Nikon system sets the power of each external group about a stop and a half low on purpose, to prevent overexposure where two or more groups may be overlapping their light (like in the studio shooting portraits).

So, if you are using only a single wireless flash, you should set your flash compensation to about
+1.3 ev as a starting point.


Jonas said...

I am using a D60 (whithout commander) and an attached SB-900 as a master flash, and a SB-600 as remote.

I discovered that the compensation is about 1.7 ev as you say, but the master flash seems to be uncompensated. Hence, the groups A, B, and C are compensated but M are not.

Am I right?

Russ MacDonald said...

Hi Jonas,

You may very well be correct.

The reason I don't know is that I never allow the master flash to contribute to the exposure, so I have not tested it.

I realize that when you have a camera without a built-in commander, you have to use an external master in the hot shoe, and you may not own three flashes, so you probably have to allow the master to contribute.

I normally use an SU-800 as a controller on my D3 and I use at least two SB-800s in umbrellas.

Maybe your post will benefit someone else who does use the master to contribute to the exposure.

Thank you,


realthk said...

Hi Russ,
first thank you for this blog, I've learned a lot here, and I don't think there's any other site with so good explanations how to use Nikon flashes.

I've recently switched from D90 to D300, and experienced the same you describe here: when my SB-800 or SB-600 is used wireless, I get strong underexposure. I'd say "at least +2/3" FV compensation required to have similar results to the same flash used on the hotshoe, but you're probably right, and it could be even more than a full stop.

It's relieving to learn this is normal and not a strange misbehaviour of my D300, but still find it strange as I don't recall anything like this from the time I used my D90...
I don't have it any more, so cannot check it, but I do believe even if it was also underexposing, it couldn't be as strong as on the D300.

Maybe I'm wrong of course and D90 was like this, but do you think all Nikon bodies behave the same in this respect?

It does not really make sense to underexpose when there's only one remote flash used.
(or is it perhaps that the camera does not know how many flashes might be in a single group?)


Russ MacDonald said...

Hi Henrick,

It appears that some cameras underexpose while others do not. I think it may be a calibration issue, but I have never sent one back to Nikon to have it adjusted.

I see no reason why it should underexpose when using only a single on-camera flash, but that's exactly what my D200 does. On the other hand, my D3 hits the exposures almost perfectly.

Thanks for the compliments!


Anonymous said...

Thank you for your superb blog! One of the finest corners on the Internet.

You said that the order in wireless mode is this:

4. Shutter opens
5. Commander fires another control sequence

Doesn't this mean then that the control sequence affects the final image? I would have thought that the commander fires its last control sequence, then the shutter would open, and then all flashes would fire.

Can you please comment on this? Thank you!

Russ MacDonald said...

Hi Anonymous (I wish you'd sign your name),

Thanks for the nice feedback!

You are absolutely right. The last command (the Fire Command Sequence) is sent to the remotes after the shutter opens, and yes, it can affect the exposure.

However, the command pulses are very low power, so usually, the effect of the fire sequence on the exposure is negligible.

But, if you are very close to your subject, then you can sometimes see its effect. Often in a portrait you can see an extra small catchlight in the subject's eyes from the fire sequence.



RWC said...

I was shooting a family portrait last night and had some very strange behaviors running wireless and am hoping you might have some ideas what was causing it. I had an SB900 in group A shooting through a trigrip and an SB600 in group B shooting through an umbrella, using the pop-up on my D300 as the commander with no contribution from the pop-up. I had the SG-3IR on the camera as well to block its reflection from a picture in the background. Ambient light was very low and the room is entirely white so the command signals should have been bouncing all over the place.

I worked to get our marks set up and the compensations correct (what a pain when you have to be in the picture yourself) before having everyone change into our photo clothes. Everything was working well and reliably.

Photo clothes were darker so I expected that I might have to tweak compensation but expected iTTL to make most of the adjustment automatically (camera was ISO 200 f/8 1/6 M mode, not that it matters). Suddenly the SB600 stops hearing the "stop" command and fires 1/1, blowing everything out of the water. I tried removing the SG-3IR, I tried putting an SB900 on the hotshoe as a commander, aimed right at the SB600's IR window, nothing worked. I put the SB900 commander on an SC-29 cable to move it closer to the SB600 and that didn't help either. I swapped out the 600 and put the 900 from the camera in the umbrella as group B remote. I still couldn't get it to stop with the SG-3IR in place but at least it worked reliably without it. Obviously, if I owned an SU-800, I would have tried that but that wasn't an option. In the end, I got the it to work with the 2 900's and the pop-up unshielded and just photoshopped out the reflection from the pop-up.

That said, I'm stumped as to what might have caused the "malfunction". Got any ideas? This was really my first chance to work with more than one light and I've never seen this problem working with just the SB600 off the camera.

Russ MacDonald said...

Hi Richard,

I assume you are aware that dark clothing will cause an increase in flash power. I have seen increases up to 1 to 1.5 stops compared to light colored clothing.

However, even 1.5 stops wouldn't be enough to totally blow out the image. It will blow out faces, though.

Maybe the SB-600 stopped working correctly?

Or maybe you accidentally changed its group to Manual mode?

One more thing that jumped out at me. You were at ISO 200 and f/8. In my experience, the SB-600 doesn't have enough power to blow out the image at those settings when in an umbrella.

You also mentioned that your shutter was at 1/6th sec. That's extremely slow for flash photography. Unless your room was extremely dark, the ambient would have been contributing strongly to the image. Is it possible that it was the ambient added to the flash that was blowing out the image?

The SB-600 may not go to as low a power setting at minimum as the SB-900 (I'm only guessing as this is not spec'd).

Why would you set your shutter so slow?

The fact that it did not work with the SG-3IR was probably not significant. It does reduce the power of the command signals. However, I have never had that problem indoors with white walls and ceiling unless the room was very large (ie, bigger than 20x20 and with 18 foot ceilings).


RWC said...

Hi Russ,

Thanks for your reply. I fear I may not have explained my question clearly enough. The issue wasn't that my images were blown out, per se, but that the SB-600 wasn't hearing the stop command. I know this because it was giving me the 3 error beeps and was clearly firing at 1/1 when that happened. When it got the stop command, it was clearly firing at less than 1/1, even with the dark clothing. Neither speedlight was being asked to do anything near full power.

The room was very dark and the point of the 1/6 shutter was to get the Christmas tree lights to appear in the background. Unfortunately the room is pretty much as you described as worst case: 2-story ceiling and 16x20.

I was using the 600 as the accent light through the shoot-through umbrella and was running with it at -1.7ev.

I did eventually get the shot I wanted, but not without some seasonally-inappropriate swearing at the equipment, as you can see at


Russ MacDonald said...

Hi Richard,

I looked at your picture and it is very nice, but of course, I can't really tell much from a small jpeg like that.

However, I do know that at f/8, -1.7 ev, ISO 200, and in a reflecting umbrella, the SB-600 is often not powerful enough to supply the required amount of power that the flash computer calculates. I think that that's why it was beeping 3 times - not because it wasn't receiving the 'stop' signal. In fact, there is NO 'stop' signal in the Nikon CLS.

Dark clothing makes the problem worse, because the flash computer tells all the speedlights to fire at a higher power level than normal. The SB-600 at -1.7 ev was probably on the brink of maxed out with light colored clothing, and when you put the dark clothing in the picture, it did not have enough power to supply what was being requested. However, the clothing in the image is really not that dark, so it probably did not increase substantially over the lighter colored clothing.

The speedlight will not give the 3 beeps if it doesn't receive the command that tells it to fire at a power higher than what it can deliver.

In the Nikon CLS there is no 'quenching' signal; ie, no 'stop' command.

The flash computer calculates the power that each flash needs to fire at, and sends the setting out to the flash during part of the preflash sequence. Each speedlight sets itself to the power that was sent to its group and waits for the 'Fire' signal.

The 'Fire' signal is the last signal sent out by the Commander, right after the shutter opens fully, and that fires all the speedlights at the power they have stored.

If a speedlight has been asked to fire at a power higher than it has available, then it fires at full power followed by the 3 beeps.

That said, a speedlight can also make errors receiving the preflash commands if the room is too big. Your SB-600 may have been making errors, but, no matter what else was happening, at f/8 and ISO 200 in an umbrella, down -1.7 ev, I'm pretty sure the SB-600 was out of power.


RWC said...

Thanks, Russ, your explanation makes sense to me now (not sure why it wasn't last night). That would explain why the SB-900 was happier.

Of course the real issue is that the SB-600 only THOUGHT it was out of power. It definitely was not. When it fired at full power and gave the 3 beeps, it totally blew out faces. The whole process was made more difficult because I had to lock the tripod down and couldn't center up on the faces, hit FV lock, and then fire because I had to be in the fool picture myself. All the more reason to continue explaining to my wife why I have to be behind the camera and not in front of it! :-) (I hate having my picture taken).

Again, thanks for your insight, both on my specific problem and in general in your blog posts. They have helped me tremendously and have helped me work out what was going on in this case and what I would have needed to do to fix this case.

Have a safe and happy 2011.

RWC said...

Also, I just noticed that you're located in the Savannah area. By way of a "thank you", and if you're interested of course, I'll get you in to see the sims when I'm next there for recurrent.

Russ MacDonald said...

Hi again Richard,

Yes, I would be very interested to see the sims at SAV. I didn't remember (or maybe I never knew) that you were a pilot. I'm a CFII with Coastal Empire Flight Training out of SAV and LHW.

Please email me directly on this at (since it's slightly off-topic for my blog - lol) .



Alex Yeap said...

Hi Russ,

Thanks for creating a great website on flash.

You mentioned that:
4. The shutter opens.
5. The Commander fires another control sequence telling all groups to fire at the power decided in step 3.

Wouldn't the control flash sequence be capture by the sensor and taken into the image exposure since the shutter already opened?

Russ MacDonald said...

Hi Alex,

You are exactly right! The Fire Command DOES ADD TO THE EXPOSURE if you don't filter the flash pulse.

This is why Nikon makes an IR filter to cover the commander in the camera (SG3IR Panel). It sits in the hot shoe and the panel part flips down in front of the pop-up flash to block the white portion of the light and allow the IR portion to pass. Only the IR portion is used by the Remote speedlights to control them.

It is the white light portion of the flash pulse that will add to the exposure. The IR part doesn't affect the exposure.

If you are using an external speedlight as a Commander, then you have to buy a gel IR filter that passes IR and blocks white light, that you attach to the front of the speedlight.

However, all that said, the Fire Command pulse is much lower power than the main flash pulse, so you can usually ignore it. I never worry about it in my wedding and portrait business.

The only time it becomes a problem is when shooting macros where the Commander is really close to the subject. This is why Nikon made the SU-800 Commander. It has a built-in IR filter and puts out no white light.

Very astute question!


aaron said...

hi, Russ if i'm using old nikon lens with no d for distance does it affect the ttl in system?

Russ MacDonald said...

Hi Timothy,

The lack of the lens distance info won't affect regular TTL, because the flash power in regular TTL is set entirely by the reflected energy in the monitor preflash.

However, the lack of distance infor when shooting TTL-BL will have serious consequences, since about 90% of the power setting information in TTL-BL comes from distance. The monitor preflash is used only as a minor input. I think it will make TTL-BL less reliable.

It may be best to stay in regular TTL at all times when using a non-D lens.


Scott Campbell said...

You said:
"I believe that this is why each group alone is purposefully set to underexpose the subject. Then, when multiple flashes are used, like in a studio, the subject is usually exposed properly."

Which means to me that I should not leave a Group on in my master if I am not using any flashes in the Group (I sometimes turn them off at the source rather than the remote). I suspect that the software will not detect the absence of preflashes.

Russ MacDonald said...

Hi Scott,

Yes, I believe it is best to leave groups that are not in use turned off. There are actually two reasons. Proper exposure is one, and you get extra delay between shutter release and flash for each group that is in use.


jmin said...

Hi, Russ. I found your blog and am reading many times and learning a lot. This time, I want to be really good at flash photography.
I have a dumb question.
I read "6. Sequence of Events for TTL Flash" and others, and I do not understand why the term TTL(Thru the lens) is included in the TTL flash. I noticed in #6 blog, the flash output level is not determined at all by the 'thru the lens' process. Am I missing something here? Thank you very much. Jason.

jmin said...

Hi, Russ. I found your blog and am reading many times and learning a lot. This time, I want to be really good at flash photography.
I have a dumb question.
I read "6. Sequence of Events for TTL Flash" and others, and I do not understand why the term TTL(Thru the lens) is included in the TTL flash. I noticed in #6 blog, the flash output level is not determined at all by the 'thru the lens' process. Am I missing something here? Thank you very much. Jason.